June 13, 2013

How The Universe Stacks Up

A major field of study in astronomy seeks to understand how galaxies form and evolve. One advantage that we possess is that as we study the light from the most distant galaxies, we are effectively looking back in time, seeing the oldest galaxies in the Universe when they were still young. But there is a problem: the farther away the galaxies are, the more difficult they are to study.

This is the issue faced by astronomers seeking to study the reservoir and relative quantity of hydrogen gas throughout the evolution of galaxies. “Distant, younger, galaxies look very different to nearby galaxies, which means that they´ve changed, or evolved, over time,” says Jacinta Delhaize, a Ph.D. candidate at the International Centre for Radio Astronomy Research. “The challenge is to try and figure out what physical properties within the galaxy have changed, and how and why this has happened.”

Astronomers predict that early galaxies contained higher quantities of the lightest element. Delhaize explains, “Galaxies in the past formed stars at a much faster rate than galaxies now. We think that past galaxies had more hydrogen, and that might be why their star formation rate is higher.”

Searching for the telltale signal that hydrogen is present is difficult because of the great distance at which these objects appear, making individual estimations of the hydrogen content difficult. To overcome this, Delhaize and her colleagues used a stacking analysis to improve the statistics of their study.

Essentially, this involves taking the data from all of the similar type galaxies in question and combining everything into a single data set. The result is that the signal in question will be more prominent against the background. While this will not reveal information about any particular galaxy, it will give an average value for the hydrogen content for galaxies of a certain age.

“What we are trying to achieve with stacking is sort of like detecting a faint whisper in a room full of people shouting,” said Delhaize. “When you combine together thousands of whispers, you get a shout that you can hear above a noisy room, just like combining the radio light from thousands of galaxies to detect them above the background.”

This work is really the first step in creating a complete picture of how hydrogen has driven star formation in galaxies over the lifetime of the Universe. Forthcoming experiments such as the international Square Kilometer Array (SKA) and CSIRO´s Australian SKA Pathfinder (ASKAP), will allow researchers to map an even larger part of the sky and over a greater range of eras.

Ultimately, astronomers hope that studying individual element abundances, particularly hydrogen, will help reveal the secrets of how galaxies form and evolve over time.